1. Field of the Invention
The present invention relates to a nitride semiconductor light emitting device, and more particularly, to a high quality nitride semiconductor light emitting device having high internal quantum efficiency but low operating voltage.
2. Description of the Related Art
To date, Light Emitting Diodes (LEDs) or Laser Diodes (LDs) based on group III-V nitride semiconductor (hereinafter will be referred to as “nitride semiconductor” in short) are often used in light emitting devices for generating a blue or green wavelength of light, and applied as light sources of various products such as an electronic display board and lighting equipment. Such nitride semiconductor is made of a GaN based material having a composition of InxAlyGa(1-x-y)N, where 0≦x≦1, 0≦y≦1 and 0≦x+y≦1. To manufacture such a nitride semiconductor light emitting device, a nitride semiconductor light emitting structure is produced by sequentially forming an n-nitride semiconductor layer, an active layer and a p-nitride semiconductor layer on a growth substrate of for example sapphire.
An active layer in the light emitting structure may have a multi-quantum well structure. The active layer is composed of a plurality of quantum well and quantum barrier layers are stacked alternately on each other to form and thus a plurality of quantum wells exist at a small width in the active layer. With charge carriers (e.g., electrons and holes) easily captured in the quantum well structure, electron-hole recombination is more likely to take place in the active layer.
FIG. 1 is a diagram schematically illustrating an energy band structure of a conventional nitride semiconductor light emitting device. Particularly, FIG. 1 illustrates an energy band diagram about an active layer in a light emitting structure. Referring to FIG. 1, an active layer having a multi-quantum well structure is formed on an n-nitride semiconductor layer, and a p-nitride semiconductor layer is formed on the active layer. The active layer includes first to fifth quantum well layers 1st QW to 5th QW, which are arranged sequentially from the side adjacent to the n-nitride semiconductor layer, and quantum barrier layers of a relatively greater band gap alternating with the quantum well layers. The number of quantum well layers may be adjusted if necessary. Typically, a quantum well layer has In density set higher than that of a quantum barrier layer and thus its band gap is lower than that of the quantum barrier layer. The band gap (or the In density) of the quantum well layer is varied according to the wavelength of emitted light.
In a light emitting device having an active layer of such a quantum well structure, the band gaps of the quantum well layers are set to be substantially uniform. Then, the lattice constant of the active layer (particularly, the quantum well layer) shows a great difference from that of the n-nitride semiconductor layer. Such lattice mismatch causes a considerable amount of strain in the active layer. The strain in turn generates a piezoelectric field in the active layer, which increases the distance between electron wave function and hole wave function but lowers internal quantum efficiency. As a result, the overall brightness of the semiconductor light emitting device is lowered.
In addition, the strain in the active layer acts to degrade the uniformity of energy level in each quantum well. Accordingly, in application of high supply voltage, charge carriers are more likely to occupy higher energy level in the quantum well. Such band-filling phenomenon is a factor that creates blue shift, in which the wavelength of emitted light is shortened in application of high supply voltage, thereby causing the emitted light to have a wavelength beyond a designed range.